Status of Collective Thomson Scattering Experiment at Frascati Tokamak Upgrade (FTU)

A diagnostic experiment of collective Thomson scattering (CTS) on thermal density fluctuations designed to provide spatially-resolved measurements of the ion temperature is since long being carried out in the high-field tokamak FTU using a high-power 140 GHz gyrotron as a source of probing radiation. A peculiar feature of CTS experiment on FTU is that the propagation at 140 GHz is below the fundamental electron cyclotron (EC) harmonic (195 GHz-220 GHz). With the only exception of the low-power experiment attempted in TFTR, this is the first proof-of-principle experiment of CTS below the EC resonance, hence with accessibility from the low-field side of both ordinary and extraordinary plasma modes. Thus, this is the only experiment capable for an exhaustive demonstration of CTS in the configuration relevant for alpha CTS in ITER, in which the extraordinary mode at 50-60 GHz propagating below the EC resonance is assumed as most liable candidate. In spite of long lasting efforts to detect CTS from thermal density fluctuations, strong anomalous spectra including spectral lines were systematically detected both in aligned- and in misaligned-antenna conditions at spectral power densities orders-of-magnitude higher than those predicted for the thermal CTS spectra. In the present communication, we report on the interpretation of the anomalous spectra observed at FTU following the results of two experimental campaigns of 2004 and 2005 expressly performed to investigate the anomalous spectra.